CA1076952A

CA1076952A - Process for the separation of dry particulate matter from a hot gas

Info

Publication number: CA1076952A
Application number: CA262,967A
Authority: CA
Inventors: Maarten J. Van Der Burgt; Pieter Buiter
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1976-01-19
Filing date: 1976-10-07
Publication date: 1980-05-06
Also published as: JPS616688B2; FR2338069A1; ZA767533B; DE2657724A1; IT1070828B; NL7600481A; DE2657724C2; BE849497A; AU503055B2; DD127761A5; IN143184B; AU2071876A; PL107984B1; GB1558008A; FR2338069B1; JPS5288871A

Abstract

A B S T R A C T
Process for the separation of particulate matter from a hot gas. The greater part of the particulate matter is removed by a cyclone. The gas is scrubbed with water in at least two stages. The aqueous suspension of particulate matter obtained in the first stage is partly injected into the feed gas. The aqueous suspension of particulate matter obtained in each consecutive stage is partly recycled to this stage and partly to the preceding stage. The aqueous suspension of the particulate matter which is recycled to the final stage is first cooled to a temperature from 25 to 200°C.

Description

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The invention relates to a process for the separation of dry particulate matter from a hot gas.
Such a process is known from Canadian Patent 1,018,328, issued October 4, 1977,J. E. Naber et al. According to this process the greater part of the particulate matter is first removed from the gas by at least one cyclone. Subsequently, the gas is scrubbed with water in at least two stages, the aqueous suspension obtained in each consecutive stage being passed to the preceding stage. The aqueous suspension of particulate matter obtained in the first stage is ; partly injected into the feed gas. The remainder of this suspension is cooled in a cooler and then recycled to the final scrubbing stage. The aqueous suspension obtained in the first stage has the highest concentration of parti-culate matter in comparison to the suspensions obtained in the subsequent stages, while the aqueous suspension obtained in the final stage has the lowest concentration of parti-culate matter.
Because the aqueous suspension with the highest concentration of particulate matter is cooled in the cooler, the latter is exposed to fairly considerable erosion.
This is a disadvantage of the process according to the afore-mentioned Canadian Patent. It has now been found that this disadvantage can be overcome by not applying the necessary cooling to the suspension originating from the first gas "

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scrubbing stage but on that of the final gas scrubbing stage, while the aqueous suspension obtained from each consecutive stage is not passed entirely to the preceding stage but is also partly recycled to the stage from which it originates.
The invention therefore relates to a process for the separation of dry particulate matter from a gas having a temperature in the range from 100 to 500C, the greater part of the particulate matter first being removed from the gas by means of at least one cyclone and the gas sub-sequently being scrubbed with water in at least two stages, while the aqueous suspension of particulate matter obtained in the first stage is partly injected into the feed gas, upstream of the at least one cyclone, characterized in that (a) the aqueous suspension of particulate matter obtained in each consecutive stage is partly recycled to the said stage and partly to the preceding stage, and (b) the aqueous suspension of the particulate matter which is recycled to the final stage is first cooled to a temperature in the ' range from 25 to 200C.
In principle, all gases which contain particulate matter can be purified by the present process. This process is particularly suitable for the purification of synthesis ` gas prepared by means of partial oxidation of oil and/or coal. In addition to hydrogen, carbon monoxide, carbon ~ . .
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dioxide, light hydrocarbons and water vapour, this gas also contains soot and/or ash and generally gaseous by-products, such as sulphur compounds and traces of HCN. The soot and/or ash particles are completely removed from the synthesis gas by means of the present process.
In order to achieve a satisfactory result by means of the present process, two requirements must be met:
~a) the water of the aqueous suspension which is injected into the feed gas must evaporate substantially completely after injection;
b) the scrubbing water with which the gas is scrubbed must remain approximately the same in volume as it circulates through the scrubbing stages.
The first requirement is met if the feed gas has such a temperature that the temperature of the gas/water vapour mixture is above the dew point of the said gas after the aqueous suspension of the particulate matter has been injected therein. The temperature of the feed gas is therefore at least 100C. If this temperature were too high, too much scrubbing water would evaporate during the scrubbing treatment of the gas. For that reason the temperature of the feed gas is not higher than 500C.
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The second requirement is met if the quantity of water which evaporates after injection of the aqueous suspension into the gas recondenses in one or more of the following scrubbing stages. This is achieved by pre-cooling the aqueous suspension of particulate matter which is recycled to the final stage to a temperature in the range from 25 to 200C. If the suspension to be recycled is not cooled far enough, too much water is discharged with the purified gas and it is necessary to add water from outside to one or more scrubbing stages in order to make up the dis-charged water. After the gas has been purified in the manner described above, it is preferably dried, This may advantageously be done by cooling the gas and separating the wàter which then condenses. As stated above, after the greater part of the particulate matter has been separated by one or more cyclones, the gas is scrubbed with water in at least two stages. To this end the gas is intimately contacted with water. This may be effected by means of one or more venturi tubes, distributing trays, packed columns or nozzles. In each stage the gas and the resultant aqueous suspension of particulate matter are separated. The gas is passed to the following stage and the suspension is divided. One part i9 recycled to the stage from which it originated.

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,., -- ~0769S2 The other part is passed to the preceding stage. The gas which has passed the cyclone or cyclones is contacted in the first stage, preferably in a venturi tube, with an aqueous suspension originating partly from the second stage and partly from the first stage. The first stage therefore preferably comprises a venturi tube. After the gas has passed the venturi tube in the first stage, the resultant mixture of gas and aqueous suspension is separated in a separator into a partly purified gas and an aqueous suspension of particulate matter which is partly recycled to the venturi tube of the first stage and partly injected into the feed gas. This contact between the hot feed gas and recycled aqueous suspension upstream of the cyclone or cyclones is preferably also effected in a venturi tube in which the water evaporates from the suspension, after which the gas/water/vapour/
particulate matter mixture is passed to the cyclone or cyclones in order to separate the greater part of the particulate matter.
The gas originating from the separator of the first stage is passed on to the second stage and is preferably scrubbed in this second stage in a gas scrubber countercurrently with an aqueous suspension of partlculate 'J matter. The gas scrubber preferably consists of a packed bed of filler elements, such as Raschig rings or Berl saddles.
A part of the aqueous suspension of particulate matter originat-ing from the gas scrubber is pumped to the venturi tubes .

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and another part is cooled to a temperature in the range from 25 to 200C and recycled to the gas scrubber. The purified gas leaving the gas scrubber is preferably cooled to a temperature in the range from 40 to 120C, so that any water vapour remaining therein condenses. The resultant water is then separated from the gas.
The invention is further elucidated in an embodi-ment in which there are two scrubbing or contacting stages by reference to the diagrammatic drawing in which auxiliary - 10 equipment such as valves, control instruments and the like is generally not shown.
A hot crude product gas containing particulate matter is passed through a line 1 to a venturi tube 2 where it is contacted with a suspension of particulate matter in water which is passed to the venturi tube through a line 3. In the venturi tube all the water in the suspension evaporates and a gas/water vapour/
particulate matter mixture is passed through a line 4 to a cyclone 5, Here the greater part of the particulate matter is separated from the mixture and removed through a line 6. The remainder of the particulate matter is passed through a line 7 together with the gas/water vapour mixture to a second venturi tube 8 and here contacted with an aqueous suspension of particulate ,~
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~0~769S2 matter which is supplied through a line 9. The mixture of gas and aqueous suspension which consists of gas, water vapour, water droplets and particulate matter which is formed in the venturi tube 8 is passed through a line 10 to a separator 11. Here an aqueous suspension of particulate matter is separated from a gas/water vapour mixture. The suspension leaves the separator through a line 12 and is passed through a line 13, a pump 14 and a line 15 to the lines 9 and 3. The gas/water vapour mixture is passed through an opening 16 at the top of the separator 11 to a gas scrubber 17 where it is contacted counter-currently with an aqueous suspension. This suspension is supplied through a nozzle 18 and a line 19 to the gas scrubber 17, and removed therefrom through a line 20. The suspension removed through the line 20 is partly passed through a pump 21 and a line 22 to a.cooler 23:~ The remainder of this suspension is passed through a line 28 to the line 13. In the cooler 23 such an amount of heat is -preferably withdrawn from the suspension that the gas/
water vapour mixture leaving the gas scrubber 17 through a line 24 has a temperature equal to the dew point of the crude feed gas supplied through the line 1. In this . way the amount of water vapour which condenses in the gas scrubber 17 will be equal to the amount of water which condenses in the venturi tubes 2 and 8.
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The gas/water vapour mixture which has been stripped of particulate matter is passed through the line 24 to a cooler 25 where it is so cooled that the greater part of the water vapour which it contains condenses and is separated in the said cooler. The dried gas is removed through a line 26. The condensed wa'er is discharged through a line 27.
The invention will now be further elucidated with reference to an Example.
EXAMPLE
In a process as described above with reference to the figure, 24,866 kg/h of crude synthesis gas was treated. This gas originated from a coal gasification unit, had a temperature of 2~3C and contained 1,462 kg/h of water vapour and 657 kg/h of ash particles. In the venturi tube 2, suspension was added to this gas at a rate of 803 kg/h, which suspension consisted of 730 kg/h of water and 73 kg/h of ash particles. In the venturi tube 2 all the added water evaporated and the ash particles were absorbed in the gas/water vapour mixture. This mixture ~s passed to the cyclone 5 where it was separated into 657 kg/h of dry matter and 25,012 kg/h of gas/water vapour/particulate matter mixture containing 22,747 kg/h of synthesis gas,

2,192 kg/h of water vapour and 73 kg/h of particulate ~ .

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'':' 10 7~9S2 matter. The dry matter was removed from the bottom of the cyclone, the gas/water vapour/particulate matter mixture issued from the cyclone at the top.
This mixture, which had a temperature of 159C, was subsequently contacted in the venturi tube 8 with 43,892 k~/h of suspension, which contained 39,902 kg/h of water and 3,990 kg/h of particulate matter. The mixture of synthesis gas, water droplets, water vapour and particulate matter which formed in the venturi tube was passed to the separator 11 and there separated into 43,528 kg/h of suspension of particulate matter in water and 25,376 kg/h of gas/
water vapour mixture. A temperature of 134C prevails in the separator. The suspension was partly recycled to the venturi tube 2 and partly to the venturi tube 8.
The gas/water vapour mixture was passed into the gas scrubber 17 at the bottom, where it was scrubbed . countercurrently with a downwardly flowing aqueous suspension. The aq~ieous suspension entering at the top of the gas scrubber had a temperature of 80c.
. Aqueous suspension was separated from the bottom of the gas scrubber at a rate of 16,577 kg/h, which suspension contained 10 kg/h of particulate matter.
Of this suspension 15,410 kglh was recycled to the top of the gas scrubber and 11,760 kg/h together :i~

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with the suspension from the separator 11 was partly passed to the venturi tube 2 and the remainder to the venturi tube 8. The aqueous suspension recycled to the top of the gas scrubber was first cooled in the cooler 23 from a temperature of 130C to a temperature of 80C. 24,209 kg~h of gas/water vapour mixture issued from the top of the gas scrubber at a temperature of 117C and was passed to the cooler 25 where it was cooled to 40C. At this stage 1,406 kg/h of water condensed and was separated. A mixture of 22,747 kg/h of gas and 56 kg/h of water vapour was removed to be further purified of sulphur compounds and HCN. This mixture no longer contained any particulate matter.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A process for the separation of particulate matter from a gas having a temperature in the range from 100 to 500°C, the greater part of the particulate matter first being removed from the gas by means of at least one cyclone and the gas subsequently being scrubbed with water in at least two stages, while the aqueous suspension of parti-culate matter obtained in the first stage is partly injected into the gas upstream of said at least one cyclone, characterized in that (a) the aqueous suspension of particulate matter obtained in each consecutive stage is partly recycled to the said stage and partly to the preceding stage, and (b) the aqueous suspension of the particulate matter which is recycled to the final stage is first cooled to a temperature in the range of 25 to 200° C.

2. A process as claimed in claim 1, characterized in that part of the aqueous suspension of particulate matter obtained from the second stage is contacted in a venturi tube with the gas originating from the at least one cyclone.

3. A process as claimed in claim 2, characterized in that the mixture of gas and aqueous suspension originating from the venturi tube is separated in a separator into a gas and an aqueous suspension of particulate matter which is partly recycled to the venturi tube and partly injected into the gas upstream of said at least one cyclone.

4. A process as claimed in claim 3, characterized in that the gas originating from the separator is scrubbed in a gas scrubber countercurrently with an aqueous suspension of particulate matter, a part of which is recycled to said gas scrubber.

5. A process as claimed in claim 4, characterized in that the aqueous suspension of particulate matter originating from a lower part of the gas scrubber is partly recycled to the venturi tube and partly cooled and then recycled to an upper part of the gas scrubber.

6. A process as claimed in claim 3, wherein said suspension injected into the gas upstream of said at least one cyclone is contacted with said gas in a second venturi upstream of said cyclone.

7. A process as claimed in claim 1, wherein said gas is scrubbed in two stages, in which a first stage comprises a venturi tube and a separator wherein a mixture of gas and aqueous suspension passes from the venturi tube to the separator; and a second stage comprises a gas scrubber in which gas originating from the separator of the first stage is in countercurrent flow with an aqueous suspension of particulate matter.